Easy keepers—horses that remain rotund despite restricted diets and rigid exercise plans—must be managed carefully to prevent or minimize more serious health issues. Equine metabolic syndrome (EMS), one condition associated with obesity, can have a serious negative impact on horses' health. Fortunately, over the past few years, veterinarians have made great strides in diagnosing and managing the disorder.

“Complex genetic tendencies interact with the environment to drive development of equine metabolic syndrome (EMS),” reported Ray Geor, BVSc, PhD, Dipl. ACVIM of Michigan State University’s College of Veterinary Medicine. He addressed a veterinary audience at the 2013 American Association of Equine Practitioners' Convention, held Dec. 7-11 in Nashville, Tenn.

Geor said that certain breed of horses and ponies appear to be predisposed to developing EMS, including Morgans, Andalusians, Paso Finos, Arabians, Saddlebreds, Tennessee Walking Horses, and Welsh ponies. On the other hand, he said, other breeds tend to be at a lower risk, including Thoroughbreds, Standardbreds, and Quarter Horses.

Horses with EMS tend to possess a body condition score of 7 to 9 on a 9-point scale; for reference, a body condition score of 1 represents an emaciated horse, where 9 represents an obese horse. Often, EMS horses have a noticeably cresty neck and localized fat deposits, especially around the sheath or mammary tissue. However, not all affected animals are markedly obese or show evidence of abnormal fat accumulations; this means that the presence or absence of obesity cannot be used as the sole criterion for diagnosis of EMS, Geor said.

One of the defining features of EMS, he explained, is hyperinsulinemia (increased insulin circulating in the bloodstream—a hormone the pancreas produces to control blood sugar levels by signaling fat, muscle, and liver cells to take up glucose). Such insulin dysregulation occurs in part because of insulin resistance–a horse’s reduced sensitivity to insulin that makes it harder for their body transport glucose out of the bloodstream. Horses with EMS, then, are subject to an exaggerated insulin response following a meal—especially when consuming feeds and forages with a higher sugar content. Season, increasing age, physical fitness level, stress or disease state, and gender also impact a horse’s insulin response.

But veterinarians’ and owners’ primary concern for horses with EMS is laminitis development. “High insulin concentrations likely contribute to the development of laminitis in an EMS horse," Geor explained. "Laminitis often occurs when affected horses have access to pasture,” as ingestion of pasture sugars and subsequently increased blood insulin concentration appear to tip the scales toward laminitis. That said, Geor notes, “Episodes of laminitis can occur when EMS horses don’t have access to pasture, so there must be other factors that trigger laminitis in these individuals.”

Something else to consider when managing EMS horses? “In obese animals with EMS, metabolic dysfunction may persist even after weight loss,” Geor relayed. This phenomenon carries over to diagnosis; since EMS can occur in nonobese horses, Geor believes veterinarians need to look for more than just the obese EMS phenotype. “Factors other than body condition score and adipose (fat) tissue mass appear to contribute to the insulin dysregulation that is central to EMS and laminitis risk,” he said.

To confirm EMS "there should also be evidence of disturbances in insulin dynamics,” he explained. “An oral sugar test is a useful diagnostic method to define insulin responses in a suspect horse. Measurement of serum triglyceride concentrations, which are mildly elevated in horses with EMS, and adiponectin, which is lower in EMS horses compared to those unaffected, are other tests that show promise for the diagnosis of EMS.”

Geor emphasized that our knowledge of EMS is far from complete. "Ongoing studies promise to shed new light on genetic risk factors as well as reasons for the abnormal insulin responses in affected animals," he said. "Improved understanding in these areas would enhance the veterinarian’s ability to identify and manage at-risk horses."

About the Author

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado,
and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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